The present invention relates to a method for manufacturing a preform for drawing out thereof an optical fiber of fluoride glass which has a low transmission loss, can be made long, and is mechanically strong. The invention is applicable to the manufacture of a preform for a fluoride glass fiber which is employed for optical communications in a 2 to 4 .mu.m band, for instance.
The use of fluoride glass is now drawing attention as a material for an optical fiber which is used for the 2 to 4 .mu.m band communications; theoretically, an ultralow transmission low of about 0.001 dB/km is expected. At present, however, the transmission loss of the optical fiber is as large as 6 dB/km even at the smallest, its length now available is also short, of the order of 100 m, and its mechanical strength is approximately 10 kpsi or so, one or more orders of magnitude smaller than its theoretical value. These shortcomings are mostly attributable to incomplete preform manufacturing techniques heretofore employed.
Conventionally, two methods, commonly referred to as a build-in casting method, and a rotational casting method have been utilized for the manufacture of preform of this kind, as disclosed in a Publication: JOURNAL OF LIGHTWAVE TECHNOLOGY VOL. LT-2, NO. 5, October 1984, PP 566-586, in the Title "Heavy Metal Fluoride Glasses and Fibers: A Review" by Danh C. Tran, George H. Sigel JR., and Bernard Bendow.
As described in detail below, the conventional manufacturing methods entail the crystallization of glass at the boundary between the core and the cladding, so that the entrapping of water in the preform results, which leads to the fiber loss, raising difficulties in making a large preform rod and in controlling the core-clad diameter, i.e. in making a preform rod for a single mode fiber. Moreover, it is almost impossible, in the prior art, to avoid scratching of the surface of the preform rod which impairs the strength of the fiber which is spun out thereof, and manufacturing steps requiring the use of human hands are involved. Accordingly, the conventional methods are low in yield rate of product and poor in industrial-scale productivity.